Abstract We have conducted a two-species pilot screen to discover new oncogenes and tumor suppressor genes that can drive prostate cancer initiation and/or progression in mice and humans. The primary screen utilized transposon-based insertional mutagenesis in mice to model genetic heterogeneity in prostate cancer and conduct a genome-wide screen for new oncogenes and tumor suppressor genes. The secondary screen used human prostate cancer patient samples to validate the relevance of novel candidate cancer genes by examining the human orthologs of the genes for altered expression in human prostate cancers. This screen identified several novel candidate prostate cancer oncogenes and tumor suppressor genes. One of the first genes identified in the mouse screen, PDE4D, was also over-expressed in human prostate cancer patient samples. Furthermore, knockdown of PDE4D reduced the proliferation of human prostate cancer cells in vitro and in vivo. This proposal will investigate the novel candidate prostate cancer oncogenes and tumor suppressor genes that have been identified in our screen with a particular emphasis on evaluating the roles of PDE4D as a candidate prostate cancer oncogene and potential drug target in prostate cancer. Experiments in Aims 1 and 2 of the proposal will model PDE4D over-expression in the normal prostate and investigate the mechanism of PDE4D action in the prostate. Experiments in Aim 3 will test NVP- ABE171, a PDE4D-selective small molecule inhibitor, as a potential anti-prostate cancer drug in the context of pre-clinical models. Experiments in Aim 4 will evaluate the expression of PDE4D and other novel candidate prostate cancer genes identified in our preliminary studies for expression changes associated with different pathologic grades of human prostate cancer and/or expression that is predictive of long-term patient outcomes. Collectively, these studies will lead to a better understanding of the genetically diverse pathways that drive prostate cancer progression, and they will determine the suitability of PDE4D as a new drug target in prostate cancer.